1. Field of the Invention
The present invention relates to a mechanism for controlling the position of an optical element in an optical apparatus, more specifically to a structure which provides an optical element holding member, movable in an optical axis direction, with a biasing force in the moving direction of the optical element holding member.
2. Description of the Related Art
In optical apparatuses such as cameras, it is often the case that a biasing force in an optical axis direction is imparted to an optical element holding member which holds an optical element and can move in the optical axis direction for the purpose of providing the optical element holding member with a role in functioning as a part of the drive mechanism for driving the optical element holding member, eliminating backlash in the drive mechanism or stabilizing the position of the optical element holding member. The biasing device for biasing the optical element holding member is usually made of an extension or compression spring which is installed so that the axis thereof extends in the optical axis direction. This configuration is disclosed in, e.g., Japanese Unexamined Patent Publication 2000-206391.
In the structure for installation of the extension or compression spring that has been widely used as a biasing device for biasing the optical element holding member, one and the other ends of the spring are respectively engaged with the optical element holding member and a separate support member (e.g., a stationary member) which is not moved with the optical element holding member so that the amount of movement of the optical element holding member directly influences on the amount of extension of the spring. The variation range of the spring load increases as the amount of extension of the spring increases.
Meanwhile, the output of the motor or actuator which serves as an element of the drive mechanism for the optical element holding member is determined to be capable of accommodating the maximum load of the biasing device for biasing the optical element holding member. Namely, the greater the maximum load of the spring member, the stronger the drive source for the optical element holding member is required, which is disadvantageous with regard to the power consumption, the production cost and miniaturization of the apparatus containing the drive source. However, in the conventional installation structure of the extension or compression spring for an optical element holding member, the spring load, which is varied according to the amount of spring extension, tends to have a large variation range, and accordingly, it is difficult to minimize the maximum spring load.
In the extension or compression spring, it is possible to achieve a reduction in load variation of the spring for a certain amount of movement of the optical element holding member by adopting an extension or compression spring having a longer length. However, in optical devices produced in recent years which are in strong demand to be miniaturized, increasing the length of the spring runs counter to space saving and thus cannot be easily adopted. In particular, in zoom lens barrels, the desire to make them compact in the lens barrel accommodated state in which no picture taking is performed has been great, and a retractable lens barrel structure in which the distances between a plurality of optical elements in the optical axis direction are minimized as much as possible to achieve a reduction of the length of the lens barrel when the lens barrel is accommodated is often adopted. Therefore, the length of the optical element holding member in the moving direction thereof is subjected to constraints of the length of the retracted lens barrel, so that it has been difficult to adopt a long spring as the biasing device for biasing the optical element holding member. As a result, the aforementioned problem of the spring load having a large variation range easily occurs.
Additionally, although the variation range of the spring load can be reduced by reducing the amount of movement of the optical element holding member, the amount of movement of the optical element holding member (namely, the amount of movement of the optical element held by the optical element holding member) is originally determined to satisfy a required optical performance, and this optical performance may not be obtained if the amount of movement of the optical element holding member is limited. For instance, in the zoom lens barrel which is constructed so as to be as small as possible in the optical axis direction when the zoom lens barrel comes into the lens barrel accommodated state as mentioned above and which is designed as a high powered lens, the amount of movement of the optical element holding member tends to be great.